Interpretive Summary: Corn (maize) is not ideally suited for growth in the southern US, because the climate is too hot with intermittent rainfall. A strategy for addressing these less than ideal conditions has been to plant corn hybrids that can vary ear size in response to environmental conditions, producing larger ears under more nearly ideal conditions. Because larger ears have more silks, they may be more susceptible to infection by mycotoxin-producing fungi that use the silks as a route to enter and infect ears. The present study was undertaken to determine if the ear size flexibility trait results in higher yields without affecting mycotoxin susceptibility. In a comparison of fixed ear, semi-flexible and fully flexible ear size hybrids, the lowest levels of the mycotoxins aflatoxin and fumonisin were found in the fully flexible ear size hybrid, consistent with the ear size flexibility trait not being associated with increased risk of mycotoxin contamination. This study should provide support for further use of the flexible ear size trait in hybrids grown under sub-optimal conditions.

Technical Abstract:
Corn (maize, Zea mays L.) hybrids expressing the flexibility trait in ear size (number of kernels per ear) are marketed for ability to give higher yields under adverse conditions. Altered kernel number is associated with altered number of silk, a major route for infection of kernels by aflatoxin-producing fungi such as Aspergillus flavus. The effect of plant density and irrigation level on yield and accumulation of aflatoxins and fumonisins in harvested grain was compared in a fixed-ear hybrid (Pioneer 33K81), a semi-flexible ear hybrid (Pioneer 3223) and a flexible ear hybrid (Golden Acres 8460) over a range of seeding densities (49,400, 61,750, 74,700, 86,450, and 98,800 seeds:ha-1) in non-irrigated, moderately-irrigated (6.4 cm soil water deficit) and well-irrigated plots (3.8 cm soil water deficit), during three years with variable rainfall. Irrigation increased yields in all hybrids, but in the absence of irrigation, yields were highest with the semi-flexible ear trait hybrid. In general, the hybrid with the flexible ear trait had lower optimal seeding densities than the other hybrids for each soil water regime. In general, kernel number was least affected by seeding density in the hybrid with fixed-ear trait compared to the semi- and flexible ear hybrids. The lowest levels of aflatoxin and of fumonisin contamination in harvested grain were associated with the flexible ear trait at all rainfall and irrigation levels, but there was no evidence that reducing stress by lowering seeding density reduced mycotoxin contamination. Inoculation with A. flavus resulted in much higher levels of aflatoxin and significantly higher levels of fumonisin contamination in grain of all hybrids under most conditions of rainfall and irrigation, suggesting that factors that promote A. flavus infection can affect production of both mycotoxins.